Feedback is a well known problem in audio systems and several systems for suppression or cancellation of feedback exist within the art. With the development of very small digital signal processing (DSP) units, it has become possible to perform advanced algorithms for feedback suppression in a tiny device such as a hearing instrument, c.f. e.g. U.S. Pat. No. 5,619,580; U.S. Pat. No. 5,680,467; and U.S. Pat. No. 6,498,858.
The above mentioned prior art systems for feedback cancellation in hearing aids are all primarily concerned with the problem of external feedback, i.e. transmission of sound between the loudspeaker (often denoted receiver) and the microphone of the hearing aid along a path outside the hearing aid device. This problem, which is also known as acoustical feedback, occurs e.g. when a hearing aid ear mould does not completely fit the wearer's ear, or in the case of an ear mould comprising a canal or opening for e.g. ventilation purposes. In both examples, sound may “leak” from the receiver to the microphone and thereby cause feedback.
However, feedback in a hearing aid may also occur internally as sound can be transmitted from the receiver to the microphone via a path inside the hearing aid housing. Such transmission may be airborne or caused by mechanical vibrations in the hearing aid housing or some of the components within the hearing instrument. In the latter case, vibrations in the receiver are transmitted to other parts of the hearing aid, e.g. via the receiver mounting(s). For this reason, the receiver is not fixed but flexibly mounted within some state-of-the-art hearing aids of the ITE-type (In-The-Ear), whereby transmission of vibrations from the receiver to other parts of the device is reduced.
Typically, feedback suppression or cancellation circuits utilise one or more adaptive filters. The adaptive filter performance is a trade-off between low steady-state error and sufficient ability to track changes. Thus, under steady-state conditions the performance is sub-optimal since the adaptive filter should be capable of adapting to a sudden change, while in dynamic situations the performance is sub-optimal because the tracking is slow.